The percentages of moisture content recommended here for wood are selected
primarily for the purpose of reducing changes in moisture content to a mini-
mir-. thereby minimizing dimensional changes after the wood is put into serv-
ice (6)2. The service conditions to which the wood will be exposed, whether
outdoors, in unheated buildings, or in heated buildings, should be considered
in determining seasoning requirements.
Timbers
Ordinarily, a timber should be seasoned to as lor a moisture content as it
will ultimately come to in service, or as near this condition as practical.
While this optimum may be possible in small and medium-sized timbers, it is
seldom possible to obtain fully seasoned large timbers. Where it is neces-
sary to use large timbers, such as in warehouses, bridges, trestles, and
derricks, some shrinkage of the assembly should be expected and the design
of the structure should take account of this condition in such a way as to
minimize shrinkage effects.
ELaterial to be used for roof trusses, arches, laminated floors, heavy plank
flooring, bridge members, derricks. and similar purposes should be seasoned
to a moisture content corresponding to service conditions, unless the large
size of the timbers makes seasoning impractical.
Lumber for Exterior or Interior Service
The moisture content requirements for finish lumber or wood products used
inside heated buildings are more exacting than those for lumber used out-
doors or in unheated buildings. This is due to the higher character of the
service required and also to the lower relative irninadity conditions encountered
within heated buildings than outdoors. Table 2 and figure 1 shor the recom-
mended moisture content values and tolerances for wood used in interior and
exterior parts of heated buildings. The values for exterior trim and siding
can be applied to lumber used outdoors and in unheated buildings.
It is the general commercial practice to kiln dry some wood products, such
as flooring (11) and furniture wood, to a slightly loiver moisture content
than service conditions demand, counting on a moderate increase in moisture
content during the storage and manufacturing periods. This practice is in-
tended to as..ure a uniform distribution of moisture among the individual
pieces. Co:mon grades and dimension are not ordinarily seasoned to the mois-
ture content values indicated in table 2. The design of the structure should
take account of this condition in such a way as to minimize shrinkage effects.
-Underlined numbers in parentheses refer to Literature Cited at end of
report.

Table 3 gives the average tangential, radial, and volumetric shrinkage for
numerous species in drying from the green condition to 15, 6, and O percent
moisture content.
In general, the heavier species of wood shrink more across the grain than
lighter ones. Heavier pieces also shrink more than lighter pieces of the
same species. Hardwoods generally shrink more than softwoods. The ratio of
total radial to total tangential shrinkage ranges from 1:1.2 to 1:2.6.
Species, however, do not alw-ays conform to the general shrinkage pattern.
For example, basswood is a light wood, but shrinks considerably more than
black locust, a heavy wood.
Values for longitudinal shrinkage are not given in table 3. The total longi-
tudinr l shrinkage of normal wood usually ranges from 0.1 to 0.3 percent of the
green dimension. Exceptionally light wood of any species tends to shrink
excessively in length.
Abnormal types of wood, known as compression wood and tension wood, also
shrink more along the grain than does normal wood. Compression wood occurs
in softwoods and tension wood in hardwoods. Longitudinal shrinkage varies
ridely-ith the form of compression w-,ood. In borderline forms that differ
only slightly from normal rood, len7hise shrinkage is but a little more
than that of normal wood. Pronounced forms, on the other hand, shrink 5 to
10 times as much as normal w ood of the conifers. In the same way, pieces of
hardoods with only a few tension-wood fibers have nearly the same lonFitudinal
shrirnikae as normal wood, but if many of these fibers are present, longitudinal
shrinkage is considerably greater than that of normal wood.
Compression wood or tension wood may occur in the same board with normal rood,
so that internal stresses are set up that cause lengthwise distortion of
boards. If the boards contain moderate to pronounced forms of compression
wood or moderate to large numbers of tension wood fibers, these stresses are
large and serious varping usually results. Even borderline forms of compres-
sion iood or tension vwood may interfere with the usefulness of pieces in
products that permit only small tolerance vith respect to warping.
Although theoretically the normal moisture content-shrinkage relation may be
considered a direct one from zero shrinkage at fiber-saturation point to maxi-
mum shrinkage at zero moisture content, actually the relationship is more like
that shPn in figure 3. For some shrirnkage calculations, however, a straight-
line relation may be assured without too great an error. For example., assume
that a piece of flat-sawed southern yellow% pine sheathing at 12 percent mois-
ture content is dried to 7 percent. According to the curve in figure 3 marked
tangential, the shrinkage from the green condition to a moisture content of 7
percent would be 5 percent while that from green to 12 percent would be 3-1/2
percent, for a difference of 1-1/2 percent. If a straight-line relation is
used, the shrink e w.:ould equal 5/30 or 1/6 of the total tangential shrinkage
of 6.67 percent, or 1.11 percent of its width.
Since the sfhinka -e values and curves represent averages, the actual shrinkage
of a board may vary somewhat from them.

partition on the first floor. This shrinkage may be sufficiently uneven to
cause plaster cracks at the junction of such ralls unless thoroughly seasoned
material is used.
If the frawe7--ork of a house is allowed to dry before the house is plastered,
c-rackint- of til ?_ plaster due to shrinkage can be reduced to some extent (10).
If dr7-wall construction is used, the framework should be allowed to dry be-
forec the, interior wall lining and interior trim are installed. PormittinC
tC sruct,-re to settle before the inner parts are installed diminishes the
dza-!a that ma-, result from the shrinking and settling. "'here plaster is
used, so:-7 i; of te moisture contained in the plaster goes into parts of the
struc+it.,rc tlat are already in place. This moisture should be removed before
finish acn, f looring are installed by heatin- and ventilating the building,
except in (iry.- hot weather.
Xh aY-/ Tim-c r Corstruction
In :.ezi ti,-Lbr construction, a certain amount of shrinkage is to be expected.
If roi ro'JIe: for in the desi-n, it ma- cause weakening of joints, affect
flor 1l!17els, an-d 1--. othei-rise objectionable. One means of eliminating part
of the srinl;-e in mill buildings and similar structures is the use of metal
1::st caps. :::2reb- the upper column is separated from the lower column only
i-,-1,e L-,al in tihe pos ca. This method eliminates the shrinkage t'hat
occurs ii the virC.er is usedbeas a bearini for the upper column. The same
tlin is r using~st pintle resting upon a metal post
C~ rvcr t hc to- of th rs lower column to support the upper column. This method
also allows the ,irdE.r to bear over the lower post. The stem of the pintle,
being encasec, is ,-rotectcd fro. fire, and as the Cirder bears over tle
column, the cap is lcss likely to fail th an if tire girder were supported
entirely by the cap.~
1-herr joist han7gers are se, the top of the joist, when installed, should be
slightly above the top cf th:e girder; othnrise when the joist shrinks in the
te floor over t!e rirder rill be hiiiher than th:at bearinc- upon the
Joist. Lamnated floor material can easily be properly seasoned and shrink-
acc( mini,:,ized aceordinlry, because each piece is of relatively small cross
-ecticn.
Interior i >L:
The normal seas;onal changes in the moisture content of interior finish are not
enou -h to clause serious dimensional change if the stock is properly seasoned
and t woerork is care;fully desig-lned and assembled. Large members, such as
ornamcxit! ars, corniace, ne'cl posts, stair strin7,ers, ands hand rails,
shol r uilt p p from comp arativellr small pieces. 'id ,,, plain surfaces,
s-ach rX t-Jl, r, o'mtcr toEs, ancd panels, shoul.I be croesbandled. Door
and vindow: trim and base should be hollow-backed. Backband trim, if mitered
at the corners, should be glued and splined before erection, othenrise, butt
joint s ,Jiould be use,1 for the w--ide faces. Large, solid pieces, such as
kniotty p p. ranchh, should be stained and finished as much as possible

Obtaining. material practically free of seasoning defects in the higher grades
of lumber is insured by adherence to approved grading rules on the part of the
..iufacturers and knowledige of the material and its grades or. the part of the
user. Defects that sometimes develop in seasoning may be classified (3) into
two main groups: (1) those caused by unequal shrinkage, wrlhich include-checks,
hone, co-b, war (fig,. 8), loosening of knots, and collapse; and (2) those
caie6d by tle ac+,ion of fungi, namely, molds, stains, and decay. Chemical
brovsn stain,, frequently known as yard or kiln brovn stain, may also occur in
some softii.oods, It is a yellow to dark-brown discoloration and is aparently
c-rsed b4 the oxidation of water-soluble materials in the wood. So-called
sticker stain is com-on in the air drying of both softvroods and hardwoods,
ancl pesurimably is also caused by the concentration and oxidation of -aater-
scl.ui rrmat rials in the wnood.
T',hese defects, with the exception of chemical stains, can be largely eliminated
byv 1ro-oer practice in either air dryin or kiln drying. Too rapidly drying
-ill cause such defects as checking and splitting, whereas too slow drying
undor favorable temperatures will cause stain or decay. The grading rules of
tjhe various lumber associations specify the amount of defects permitted for
the various grades of lumber. Most defects are specifically mentioned, but
such defects as honeycombing and collapse are covered indirectly, as for
examrznle in softwrood cradinc rules that state:
'rT 'hen defects or blemishes not described in these grading rules are encountered,
they shall be considered as equivalent to knwon defects according to their
da-;ai,.,-1 effect upon the piece in the grade under consideration,"
Hone yco; ,binF and collapse are more coLmon in hard- oods than in softivoods and
are l.,ore likely to occur during improper kiln drying than during air drying.
11oisture Content of
Seacone Lumber
T'i.,Ie trade terms "ship il"air-dry," and "kiln-dried," although -widely
used, have no specific cr agreed meaning ,:ith respect to quantity of moisture.
T1-i: .ide limitat4,ons of these trms as ordinarily used are covered in the
f'o llc:in_ statement, -hci-i, hoever, are not to be construed as exact
d(_Iinition s:
Sip:inr-dry 1=b(br.--Lirber ti-at is partially air dried to reduce freight
r ~. and -nVi have a moisture content of 30 percent or more.
Aki-r-Cr-y lur-er.--Lumber t'nat has bee.n exposed to the air for any length of
time. If CxT~osed for a sufficient length of time, it may have a moisture con-
tunt r-nginlf from 6 percent, as Jn slim-rer in the arid Southw est, to 24 percent,
's in tt w wjnt( ,r in the, Pacific ':7orthivest. For the United States as a whole,
tie mini-:11un, moist urc, content rmie of thoroughly air-dry lumber is 12 to 15
perc, nt, a-1. t Ie a-vcae is so me ,-hatl l higher.

merely by heating the air to 47* F4 In northern areas, heat is required
primarily eurinF t he winter when the outdoor relative humidity is high, and
tie comili rium moisture content of iuood exposed to outdoor air may be lc to
20 percent. If the temperature of the storage shed is kept 10' F. higher
than the outdoor temperature the lumber r-ill usually be brought to or main-
tained at a 10 percent moisture content. If the temperature is increased
20' F. the moisture content vwill be about 7 percent. During cold w-eather,
if the storage shed contains any water lines the temperature should not be
allowed to drop below. 320 F.
Care of Ljmber and Finish Durine Construction
I Ll.ber and Dimlcnsion
Ordinarily, jr'-cn lumber should not be used in the construction of a building.
JroCe s. rd, however, are comm-only used, and their crying and shrinking- during
tl cursi obf does not usually result. in much dan age. VThen Fren stud
:lat~ ial is use-d for wrall plates or caps, the resulting shrinkag7e is more likely
to be detrimental. Dry lumbc7r received at the building site should be protect-
eid against, -ettim. It may be solid piled on three timbers laid on the Pround,
and the pile covered vith roll roofin. or water-resistant. paper, Unless the
!L=,b r Jlo is prot oted a ainst precipitation, stickers about L feet apart
siold~i separate the layers of lumber.
L1~tr tiat is received in the Freen or nearly green condition, or lumber that
.as en u[ed for concrete forms, should be piled wvith stickers for more
t1.-orDu- dryin before It is built into the structure.
- ixnber, wrter dry or r-rn, should br protected from alternate wmetting by
ra--n and drying by direct susbine in order to reduce checking and iwarping.
Frequently in the construction of houses, the garage, if detached, can be
built f-rst ani l rill serve as an excellent storage space for sheathing,., siding,
sds, and joists.
12nsh loor
C; ac'i w (",vv, op in flooring if it. absorbs moisture either before or after it is
Lai( shrinks viricn tlie buildin.- is heated (11). Such cracks can be
FrrC klY reduced, if not entirely elir:inated, by observing the following
Iractjc
a:: sold b- Ualers tl- at pro-tcct it roporly; (2) do not allow, tie flooring
t ( J, cliv(, rc( on a damp or rainy day or before the masonry and plaster Valls
,_rF: -y; () Jliminate all badly crooked -picces or use them in inconspicuous
Ia~ : 'i at V builiinj .
:t r a;,l :-oo(l-lr ian-in and finis1hins can be done wi-en the house is warm
a t: 1. i, -o 7 1)as _, n k( driA (Ci- aTproximnte m 7thod of deterining whether
t :1air i n a Uui 1 l ( iY s dri- r oue ch to er1elmit the delivery and installation of
fioorin- and o i r interior voodviork is to take two readings on a -ret-an-dry-

When the heating system or portable heaters are used to prevent freezing of
plaster and to hasten its drying, the windows should be properly adjusted to
allo77 the escape of the evaporated moisture. Even in the coldest weather,
the windows on the leeward side of the house should be opened 2 or 3 inches,
preferably from the top.
Determination of Moisture Content
The amount of moisture in wood is ordinarily expressed as a percentage of the
weight of the wood when oven-dry. Three distinct methods of determining mois-
ture content are described below. The oven-drying method is probably the
most nearly exact, but is slow and necessitates cutting the -wood; the distil-
lation method is necessary if the wood contains creosote or other volatile
oils; the electrical method is the most rapid and does not necessitatecutting
the material.
Oven-drying Method
In the oven-drying method (4) cross sections, about 1 inch long in the
direction of the grain, are cut from representative boards of a lot of lumber.
These sections should be cut at least I foot from the ends of the boards to
avoid the effect of end drying, and should be free from knots and other
irregularities, such as bark and pitch pockets.
Each section is immediately weighed, before any drying or adsorption of mois-
ture has taken place, and is then placed in an oven heated to 212o to 221o F.
and] kept there until it reaches constant weight. If the section cannot be
weighed immediately after it is cut, it should be wrapped in metal foil until
it can be weighed. In the oven, a section will reach a constant weight in 12
to 48 hours. For weighing ordinary moisture content sections, balances having
a capacity of about 200 grams and sensitive to O.05 gram are recommended.
Both steam and electric ovens are in common use for drying moisture-determina-
tion sections. The sections, vth either type of oven, should be open piled
in order to permit good circulation of air, especially around the end-grain
surfaces, and thus hasten drying.
The constant or oven-dry weight and the weight of the section when cut are
used to determine the percentage moisture content following formula:
Percent moisture content -weight when cut oven-dry weight x 100
oven-dry weight

of 1 percent to the meter reading for each 20' F. below 70 F., and the sub-
tracting of 1 percent for each 20' F. above 70' F.
The electrical method's principal advantage over the oven-drying method, is
its speed and convenience. The time required to determine the moisture con-
tent of any piece of' wood is only a few seconds. It is, therefore, adaptable
to sorting lumber on the basis of moisture content, and can be used to measure
the moisture content of wood installed in a building. With the electrical
m(-ot1, the piece of wood is not cut or mutilated except for the driving of a
fev small needles into the wood to serve as electrodes for the resistance-
type meters.

O
Figure 2o--Characteristic sh-riLkage and distortion of flats., squares, and round as affected
by the direction of the annual rings. Tangential shrinkage is about twice as
great as radial,
ZM 12494 F

'e, m 1-4, / /W

Figure 3,--Typical moisture-shrinkage curves, These curves are for
Douglas-fir and southern yellow pine and may be used for
estimating the amount of change in dimension that will
take place with change in the moisture content of the wvcd
ZM 22048 F

D! i R1903-7

/ 2 3 4 5 6 7 8 9 /0
SHRINKAGL (PtR CE/Il T OF 6RrEEN 01ML/ENSION)

28
26
24
22
4
K/0
6

0

Figarcae 4,--Joists run over top of girder increase the vertical height
of the wood used across the grain and increase subsequent
shrinkage.
ZM 22072 F